1. Field of the Invention
This invention relates to thermal ink jet printing devices and, more particularly, to thermal ink jet printheads having droplet generating heating elements which are energized by packets of constant amplitude pulses in which each pulse in the packet has its pulse length and intervening time intervals varied in response to the manufacturing tolerance variation, number of parallel heating elements concurrently energized, and the printhead temperature in the vicinity of the heating elements.
2. Description of the Prior Art
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing system which uses thermal energy to produce a vapor bubble in an ink filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electric pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and the bubble starts to move toward the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resuslting in separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line towards a recording medium, such as paper.
Thus, thermal ink jet devices operate by pulsing heating elements in contact with ink so that bubbles are nucleated, ejecting ink droplets toward the paper. It has been found during print tests that print quality is affected as the device heats up. This is because the volume of the droplet and therefore the printed spot or pixel increases as a function of printhead temperature. Through study of this phenomenon, it has been found that both the mass and velocity of the droplet increase with device temperature, and that both the mass and velocity contribute to increase pixel size on the paper. For the carriage-type ink jet printer with sufficiently high printing density, the spot size increases as the carriage traverses the page. Then, as it pauses at the end of travel and reverses direction, it cools slightly, so that the next line or swath printed on the way back has increasing pixel sizes in the opposite direction. This gives rise to light and dark bands, which are most pronounced at the edges of the paper. Similarly, other patterns of high and low density printing are degraded by the increase in pixel size with device temperature.
Many of the prior art devices incorporate a heat sink of sufficient thermal mass and of low enough thermal resistance that the device temperature does not rise excessively. For one example of a thermal ink jet printhead having a heat sink, refer to U.S. Pat. No. 4,831,390 to Deshpande et al. This approach has eliminated the catastrophic printing failure mode. However, to lower the thermal resistance to the heat sink sufficiently that there is no appreciable device temperature rise in the time scale of a carriage translation in one direction across the paper, it may be necessary to take packaging approaches which would increase the cost or otherwise constrain the printer design in an undesirable way. The temperature rise must be maintained such that negligible image degradation occurs because of thermally induced spot size non-uniformities. The same printhead, but at an increased temperature, ejects a larger droplet which produces an increased spot size when printed on a recording medium, such as paper. This increased spot size may lead to observable print quality defects. Another factor which influences the energy required to vaporize the ink having the desired bubble volume is the manufacturing tolerance variation encountered for the heating elements. This is especially true for doped polysilicon heating elements.
A copending patent application Ser. No. 07/375,162, filed Jul. 3, 1989, entitled "Thermal Ink Jet Printhead With Constant Operating Temperature" to Kneezel et al, discloses means to prevent printhead temperature fluctuations during printing, especially in translatable carriage printers, by selectively energizing the heating elements not being used to eject droplets with energy pulses having insufficient magnitude to vaporize the ink. Patent application U.S. Ser. No. 07/457,499, filed Dec. 27, 1989, entitled "Method and Apparatus for Varying Pulse Duration and Power in a Thermal Ink Jet Printer to Maintain Constant Ink Droplet Size or Vary Ink Droplet Size", to Ims et al, discloses a method and apparatus for sensing the temperature of the printhead and varying the pulse parameters of the electrical signals applied to the heating element to maintain a substantially constant droplet size or to vary the size of the ink droplet. The variation of the pulse parameters includes variation of the pulse duration and voltage. These two copending applications are commonly assigned to the assignee of the present invention.
U.S. Pat. No. 4,872,028 to Lloyd discloses a thermal ink jet printing system having a drop detector which is used in a feedback loop to optimize operations drive pulse parameters of the electrical pulses supplied to the heating elements. During a maintenance procedure during startup, the test generator causes the pulse controller to test each of many drop generators with a series of fixed voltage rectangular pulses of digitally increasing pulse width. The pulse width at which a drop is first detected and the velocity of each drop detected is correlated with the width of the pulse which generated that drop. The algorithm function calculates an individual operational pulse width for each drop generator or alternatively, a common operational pulse width for all drop generators. The pulse parameter value set so determined is programmed into the pulse controller and used during normal printing operation. The advantages of pulse width as a variable notwithstanding, pulse amplitude is also a suitable variable pulse parameter. However, the control of the single pulse width and/or pulse amplitude is rather complex and expensive, and in the case of multiple printheads or individually controlled heating elements, the complexity and cost is prohibitively high.
Thermal printing, a related technology, is accomplished by raising the temperature of the thermal print medium above a threshold temperature whereupon a coating on the thermal print medium undergoes a chemical change and changes color. Typically, the temprature of a thermal print medium is raised by the use of a thermal printhead that includes one or more resistive print elements that are mounted, for example, on a ceramic substrate and that are maintained in contact with the thermal print medium. The configuration of each print element defines a portion of a character or an entire character to be printed. It is important that a thermal printer be capable of precisely controlling the amount and duration of heat to print each character portion. Control of the amount of heat applied to the thermal print medium is achieved in part by controlling the exposure time; that is, the time during which the thermal print medium is held above the conversion or printing temperature. In order to provide halftone or gray scale recording by a thermal printer, the temperature of the heating elements must be accurately controlled above a printing threshold temperature for various predetermined periods of time.
In contrast, ink jet printers must heat the heating elements to a temperature in which the liquid ink in contact therewith instantaneously vaporizes into a bubble and the duration in which the vaporization temperature is held by the heating element is minimized to the extent possible, so that the electrical pulse is immediately shutoff. U.S. Pat. No. 4,675,695 to Samuel discloses a technique whereby the electrical pulse applied to the heating element is shaped to reduce the maximum temperature of the heating elements. This is especially effective in thermal printing because the heating element must be maintained above a threshold temperature for a predetermined amount of time. The thermal printing apparatus of Samuel comprises a thermal print element and control means for providing energy at a first average rate for a time sufficient to raise the temperature to the print element from ambient temperature to a temperature above the threshold temperature and then provide an energy at a second average rate that is less than the first average rate, but nevertheless sufficient to maintain the temperature of the print element above the threshold temperature. The control means provides electrical energy to the thermal print element in response to a strobe signal which comprises a first pulse followed by a series of second pulses. The first pulse has a length sufficient to raise the temperature of the print element above the threshold temperature and the second pulse has a length shorter than the first and a series of second pulses has a duty cycle selected to maintain the temperature of the print element above the threshold temperature for a predetermined time period.
U.S. Pat. No. 4,633,269 to Mikami et al discloses a thermal printer which conducts recording by heating heat generating elements with a drive signal. The temperature of the heat generating elements after a specified period from the start of a thermals recording signal can be returned to a constant value, by applying during the normal cooling process, that is, after application of a thermal recording signal, a predetermined auxiliary pulse corresponding to the temperture that is generated during the thermal recording and to the tone to be recorded. Thus, a predetermined temperature is maintained from which the thermal print elements are pulsed thereby eliminating the affect of temperature differences resulting from stored energy which varies with the tone density required of the heating element in its previous energization.
U.S. Pat. No. 4,745,413 to Brownstein et al discloses a continuous tone thermal printer having a printhead with a plurality of hating elements. Each heating element is energized during first and second halves of a line print time interval to more uniformly distribute heat during such an interval. A storing means stores values representing a desired density of each image pixel of a line, while a means responsive to such stored numbers energizes each heating element during different portions of a time interval to cause heat produced by such heating elements to be uniformly distributed throughout the time interval to reduce line gaps.
U.S. Pat. No. 4,688,051 to Kawakami et al discloses a thermal printhead driving system which supplies a predetermined number of driving pulses to each of a plurality of heat producing elements. The pulse width of the driving pulse is controlled in accordance with the temperature in the vicinity of heat producing elements.
U.S. Pat. No. 4,345,262 to Shirato et al discloses a thermal ink jet recording method where the addressing pulse applied to the bubble generating heating elements have a specific pulse width range and the addressing cycle is at least three times as large as the pulse width.
U.S. Pat. No. Re. 32,572 to Hawkins et al discloses a thermal ink jet printhead and method of fabrication. A plurality of printheads are concurrently fabricated by forming a plurality of sets of heating elements with their individual addressing electrodes on one substrate surface and etching corresponding sets of grooves which may serve as ink channels with a common reservoir in the surface of a silicon wafer. The wafer and substrate are aligned and bonded together so that each channel has a heating element. The individual printheads are obtained by milling away the unwanted silicon material in the etched wafer to expose the addressing electrode terminals on the substrate and then the bonded substrate and wafer are diced into a plurality of separate printheads